Types of plate boundaries

          

Before knowing anything about plate tectonics we have to learn about earth’s crust and mantle. Generally earth crust is about 5-10kms thick in oceanic region, about 35kms thick in continental areas and 55 – 70kms in orogenic belts.

Below the crust there is the region which is responsible for most of the earth internal energy and the forces responsible for ocean – floor spreading , continental drift , orogeny and major earthquakes is the mantle. Mantle is about 2865kms thick. Upper part of mantle and crust together constitute a region call lithosphere.

So it is the region which is responsible for the movement of the plates. In a general way, plates are the large blocks of the rock below the earth crust which has the tendency to move .

The concept of “plate – tectonics” involves a world-wide network of moving lithospheric plates. The concept was introduced by American scientists Hess and Dietz. The thickness of plates is considered to be 10kms at the ridges and 100 -150kms elsewhere. Generally there are six major plates along with other smaller plates. These major plates are:

1)                Pacific plate

2)                American plate

3)                African plate

4)                Eurasian plate

5)                Indian plate

6)                Antarctic plate

So the plates are continuously in motion both with respect to each other and to earth axis of rotation.

Generally these plates move with velocity of about 1-6cm per year. The plate moves towards each other, away from each other or past one another. Depending upon whether the plate moves,

 Plate BOUNDARIES  are classified as three types

  A) Constructive

                  B) Destructive

                  C) Conservative 

Generally plate boundaries are the area where there is maximum geologic activities which are mainly due to the movement of the plates.

 

1)                Constructive boundaries :

These are also called divergent plate boundaries.

When two plates are in motion away from each other then it alloys hot molten mass to come upward from the mantle and forms a new surface when it cools. These types of the divergent takes place in the middle of an ocean or in the continent. So, on the basis of its occurrence constructive boundaries are divided into:

a)                 Continental divergence :

This is also called continental rupturing. During this process a large column of mantle rock begin to rise from the mantle upward to the crust.

The rock mass uplifts from the continental plate rupturing everything in its path up to the crust causing the elevation of the continent.

This column of the heated mantle rock mass is called mantle plume.

 Due to more elevation the earth crust gradually becomes thinner and thinner and the maximum tensional force causes the plates to facture forming valley like (rift valley).

 

As a result it results in basaltic volcanism along the fracture within the rift valley. The fracture slowly expands across the plate resulting the whole plate to rupture or to divide into two new plates.

 

When divergence continues further the continental plates distinctly separate from each other with the widening and deepening of rift valley which allows the oceanic water to enter between two continental plates.

Hence continental divergence forms two new continental plates by the rupturing of the continent allowing oceanic water to separate them.

 

b)                Oceanic divergence :

]The oceanic divergence in the ocean is highlighted by the mid oceanic ridge.

Even though the plates moves opposite to each other but they do not separate because of the addition of materials like magma as a result of volcanic eruption in mid oceanic ridge.

But the association of volcanism with the crest of oceanic ridge generates the tensional force which allows the lithosphere to extend.

The expansion of the surface results to slow down the pressure of volcanic magma as the surface area gradually increases due to tensional force.

The fissure opens more and more resulting the magma to solidifies and creates new oceanic floor within the rift valley.

 

 

 

2)                Destructive boundaries:

   These are also called the convergent plate boundaries.

    In this case two plates moves towards each others.  Here two plates collide with each other resulting into only one plate. Hence due to the destruction of the plate these are known as destructive plate boundaries.

It can be classified as:

a)     Ocean – ocean convergence

b)    Continent- continent convergence

c)     Ocean – continent convergence

 

 

a)     Ocean – ocean convergence:

 

When the convergence occurs between two oceanic plates it is referred as ocean – ocean convergence.

In this case, two oceanic plates converge and one plate bends downward beneath the other.

This process is called subduction. The density also varies between two oceanic plates as density of the older plate is predicted to me more than the younger.

So,older (denser) plate subduct beneath the less denser one.

During the process of subduction, the temperature gradually increases due to the friction between two plates with increase in depth.

At the depth of about 150- 200 km, the melting of the descending plate takes place due to maximum frictional heat and geothermal gradiant.

The melting of the plate results in anatexis of the rocks which leads to produce magma chambers. As the magma produced is less dense than the surrounding rocks so it tends to raise upward and at the point of maximum pressure it rupture the country rock and finally reaches the surface as a result of volcanic eruption.

At the point of maximum deposition the cone starts to uplift higher than the sea level and forms the chain of island arc on the surface.

 

b)    Continent - continent convergence:

 

This is the process where two continent moves towards each other and collides. This is also known as the continental collision.

This is considered to be the much complex process of convergence. As both the continental plates being much thicker and less dense than the lithosphere collides with tremendous effect on both plates.

Due to the density of both plates as compared to the underlying lithosphere they are mostly prevented from subduction but also one can be subducted in some cases.

 The tremendous force released during the collision results in forming tectonic structures like fault, folds, joints and also the formation of young mountain ranges.

One of the best examples to support this is the Himalaya mountain series. Himalaya mountain series is the result of collision of two continental plates (Eurasian and Indian).

 

c)     Ocean – continent convergence:

         

This is the phenomenon when both oceanic and continental plate moves towards each other.

During the convergence the thinner and more denser oceanic plate subduct beneath continental plate.

When the descending oceanic plate reaches about the depth of 150km, it gradually starts to melt due to high friction and high geothermal heat.

The melting of the rocks leads to form magma and the magma chamber up to a certain depth. The magma being less dense than the surrounding rocks starts to uplift by rupturing / penetrating the surrounding rocks.

 The magma reaches to the surface by volcanic eruption and magma chamber also uplifts as a result of isostatic uplift to form the structures like volcanic arcs.

                                   

 

 

3)                Conservative Boundaries:

These are the boundaries where two plates slide past each other along a fault. The motion of the plates are away from each other and also referred as the strike slip along the fault.